This invention relates to a power transmission and to a method of making and assembling the transmission. It relates more particularly to a unique type of mechanical power and motion transmission that operates within a very compact space envelope by causing a radially positioned group of load sharing rotor elements, when driven by a central drive worm, to rotate and precess along a multiplicity of stator races with said races forming collectively a geometric torus.
This transmission can be driven from either side to function either as a speed reducer or speed increaser with equal efficiency. With its coaxial configuration, this transmission is uniquely suited for handling high torque requirements in confined spaces where minimum weight, minimum volume, and low acoustic noise are of premium value. The applications that can be addressed with this transmission are, for example, helicopter rotors, automotive wheel drives, actuators in earth moving heavy equipment, industrial drives, tank sprocket drives, and precision positioning and motion devices.
Over the years, I have developed several power transmissions which incorporate the kinematic principles of the toroidal drive. Those early transmissions are described in detail in my patents 4,297,919 and RE 26,476, among others. Those early efforts were aimed at building highly compact mechanisms whose motion and power transmission capabilities depended, at least theoretically, on the employment of multiple, load-sharing, rotary bearing elements to transmit high torque at high reduction ratios. The fruits of those efforts were relatively compact, lightweight, high-efficiency gear reducers whose additional attributes were high-resistance against shock and geometric coaxiality between input and output.
However, when attempts were made to implement the theoretically possible load sharing abilities and to build those devices in quantity, certain serious limitations appeared such as the impossibility to assemble a full complement of rotors, and the high cost and difficulty in manufacturing some of the elements and severe restrictions with regard to the number of load-sharing bearing elements that could actually be employed in a given transmission to handle large loads on the output shaft. There was an equally serious limitation regarding the practical range of speed ratios that could be achieved between the transmission input and output. More particularly, only transmissions with gear ratios of 36:1 or more could be made as a practical matter.
A transmission of the type described in the first above patent is depicted in FIG. 1. The load-sharing elements in this type of transmission comprise rotor units 10 each of which includes a hub 12, a ring 14 rotatably mounted coaxially to the hub and a plurality of fingers 16 extending radially outward from the ring, the fingers being terminated by drive rollers 18. The rotors 10 are mounted via the hubs 12 to a large ring 20 centered on the common rotary axis of the transmission input and output shafts 22 and 24. The rollers 18 of radially inner fingers 16 of the bearing units engage in the grooves of a worm 26 connected to the input shaft 22 and the ring 20 to which the bearing units 10 are mounted is connected by arms 28 to the output shaft 24.
The outer fingers 16 of the rotor units 10 engage in grooves or races 32 inscribed in the interior of a two-section transmission housing 34. When the drive worm 26 is rotated by the input shaft 22, the various rotor units 10 are caused to rotate about their respective hubs 12. Since the rotors also engage in the stator races 32, rotation of those rotors causes the rotors to advance along the races which, in turn, causes the ring 20 to which the rotor units are attached to precess about the rotary axis of the transmission. Since the ring 20 is connected to the output shaft by arms 28, when the ring 20 rotates, so does the output shaft 24.
FIG. 1 depicts a transmission having six planetary rotor units each of which has six rolling fingers 16. However, it has become obvious that smaller gear ratio transmissions with the toroidal races 32 having generally rectangular cross sections as shown simply cannot be assembled on a production basis with more than three fingers on each rotor unit 10 because of the three-dimensional nature and spatially twisted geometry of the toroidal races 32 that have to be engaged by the rotor units 10.
More particularly, when assembling the transmission, the drive rollers 18 on the radially inner fingers 16 of all of the rotor units 10 must engage the threads of the drive worm 26 and the rollers of the radially outer fingers of the rotor units must simultaneously engage in the helical races 32 inscribed in the stator housing 34. Since the rotor units are more or less flat and the races 32 are spatially twisted or skewed at relatively steep angles, it has proven to be very difficult to locate all of the rotor unit fingers in their respective worm threads and races all at the same time to enable the upper and lower sections of housing 34 to be brought together so as to envelop the rotor units, especially when the transmission incorporates rotor units with more than two rolling fingers.
In other words, the crossover angle between the rotational plane of the rotor unit and associated helical race has to be quite large, leading to a twisted race geometry into which the drive rollers 18 of the multiple rotor unit fingers cannot be inserted during assembly of the device due to that twist. For example, the FIG. 1 transmission having a double-threaded worm 26 and six rotor units 10 each having six fingers 16 has a gear ratio of 12:1 and the rotor units rotate four times for each revolution of the output shaft 24. The helical races 32 of this 12:1 design possess such steep lead angles that the drive rollers 18 on all of the planar rotor units simply cannot be inserted into the races during assembly of the transmission due to mechanical obstructions.
Thus, the practical gear ratio restrictions imposed on those early toroidal transmissions limited their field of use as a torque-capable transmission.
To alleviate some of the above problems, a new transmission and mode of assembling same was developed and is the subject of the aforementioned pending application, the contents of which is hereby incorporated herein by reference. That transmission, like the one in FIG. 1, has two stator halves in two housing sections, i.e., each stator groove or race is split across the middle. The internal finger rollers of the transmission are assembled through special access holes provided in the groove bottoms or by fabricating the stator halves out of multiple segments arranged in the two housing sections.
To assemble that transmission, the input and output shafts are rotatably coupled together and the rotor units are mounted to the arms of the output shaft such that the drive rollers on the radially inwardly facing rotor fingers engage the central worm. At this time, however, the radially outwardly facing rotor fingers are devoid of drive rollers.
Next, the two housing sections are engaged together so that they envelop the rotor units. Only then are the drive rollers for the radially outwardly facing rotor fingers attached to those fingers through specially placed access holes provided in the two housing sections which lead from the bottoms of the stator races to the outside.
By rotating the input shaft, the end of each rotor finger may be positioned opposite one or another of the access holes so that the drive roller for that finger may be inserted through the hole and fastened to the end of that finger.
However, as noted above, that transmission is still disadvantaged by having split stator races which cause wear, vibration and noise when the transmission is in operation as the rotor drive rollers roll across the splits. Also, it has proven difficult to form the helical stator races in the usual way by "plowing" or cutting material from blocks of metal in a three-dimensional machining operation.
Still further, due to the aforesaid closed stator construction, that prior transmission is relatively heavy and bulky and requires internal lubrication to prevent the rotors and other internal parts from overheating.
As a result, these are some applications for which that transmission may not be suitable.